Field of the Invention
The invention relates to a brake system for a motor vehicle having electromechanical brake actuators assigned to wheels.
Nowadays, passenger cars are almost always provided with hydraulic brake systems. The increasing demands which are made of additional brake functions, such as anti-lock brake functions, driving stability systems and traction control systems, as well as requirements for reduction in the installation and maintenance costs and a decrease in the hydraulic devices, have led to the development of electromechanical brake systems.
In such brake systems, the driver is disconnected from the brake in terms of force, that is to say the braking torque request which emanates from the driver is not transferred directly as a force through a hydraulic system but rather only as a signal through an electrical lead. That signal is used to control an electric brake actuator which generates a force at a brake with an electric power supply and that force brings about the desired braking torque through the use of a friction element. In the case of a disk brake, the force which generates the braking torque is referred to as a brake engagement force.
In an electromechanical brake system, such as is described, for example, in German Patent DE 196 15 186 C1, corresponding to U.S. application Ser. No. 08/843,887, filed Apr. 17, 1997, the brake actuator is mounted directly on the respective wheel of the motor vehicle so that a brake engagement force is expediently not transmitted over large distances in the vehicle. The braking torque request and the brake controller are disposed at a central point, for example in the vicinity of the brake pedal. The brake system is thus distributed over the entire vehicle, and since it forms a central safety function of the vehicle, increased safety demands are made of the electrical transmission of the braking torque requests.
The braking torque request is usually tapped from the brake pedal through the use of force sensors and displacement sensors. However, such sensors have non-detectable fault states so that a number of them, for example three, are disposed in a redundant way. Therefore, even when a sensor fails, the braking torque request can still be reliably detected, for example through the use of a 2 out of 3 majority decision.
Brake systems which distribute all of the sensor signals to the brake actuators have been known to date. The majority decision is then taken in a decentralized manner in the control unit of the respective brake actuator, as in German Patent DE 195 21 175 C1. That provides a high degree of safety, but the expenditure involved, especially on the cabling in the vehicle, is considerable.